If we combine subjects in grade school, like history and geography, or spelling and phonics -- teaching them together, from one book or at the same time -- how do we report that? Would there still be two grades on the report card? Do there have to be?

For grade school, you have a lot of leeway. You can either report them as separate grades if you feel there has been enough work to warrant separate grades, or you can just call a course 'History and Geography' and give one grade.